Evaluation and Improvement of Uniformity in km-Class MgB2 Superconducting Wires

An 18-multifilamentary MgB
₂
wire, classified within the 250 m-class, was produced utilizing a technique to improve the uniformity of powder filling by employing three-dimensional vibrators. A comprehensive evaluation of the fabricated wire revealed that the uniformity of the critical current (
I
_c
) and fill factor, determined through the coefficient of variation, were 2 and 4%, respectively, along the wire direction. Building upon this achievement, a scaled-up MgB
₂
wire was fabricated, with dimensions of approximately 3 km in length and 0.83 mm in diameter. At 20 K and 2 T, the
I
_c
was measured to be 191 A. The corresponding critical current density (
J
_c
), determined to be 190,000 A/cm
²
, represents a significant improvement when compared to that yielded by the conventional gravity-based powder filling method.